Solar cells are being intensively studied in recent years since they are able to generate clean renewable energy without causing an increase of atmospheric carbon dioxide or other dangerous green house gases and thus have a strongly positive influence on the climatic balance in the atmosphere. Solar cells can also partly replace nuclear energy generation which always entails hazardous risks.
A variety of solar cell modules for commercial and home appliances have been proposed. These solar cell modules are typically constructed as a layered structure including a packed interconnected assembly of photovoltaic elements, also known as solar cells. Said layered structure includes a substrate that can be for example made of metal or glass, a semiconductor active layer disposed on said substrate, and a transparent and/or conductive layer disposed on said semiconductor active layer. A protective means is also needed in a solar cell module, to protect it from heat, humidity and from any possible environmental exposure or risk during transport. Especially in the case that the substrate is opaque, said protective film has to be transparent in order to allow the solar light to reach the solar cell module active layer.
Said protective means conventionally comprises a top coat layer made of a transparent fluorine-containing polymer comprised of a fluororesin or of a fluororesin-containing composition; often, an encapsulating layer is used under the top coat layer, in direct contact with the photoactive element.
The use of fluoropolyether compounds in coating compositions is already known.
US2006/0252910 (ASAHI GLASS COMPANY, LTD) discloses a fluoropolyether compound useful as lubricant, surface modifier or surfactant in coating compositions.
US2007/0179263 (DAIKIN INDUSTRIES, LTD) discloses a ionic liquid aromatic material useful as lubricants, acid remover agents, electrolytes for dye-sensitized solar cells and actuator materials. The ionic lyquid type functional material contains an aromatic compound that has a fluorine containing ether chain.
WO08/005327 (LIQUIDIA TECHNOLOGIES INC) discloses a photovoltaic device including an encapsulation layer fabricated from an elastomeric material such as for example a PFPE having favorable optical properties, gas permeable, scratch resistant, conformable liquid material. The encapsulation layer may also include a structured surface having the purpose of manipulating and trapping light incident on the photovoltaic device.
However, a protective film for a solar cell module simultaneously ensuring good transparency to light radiation, chemical and oxidative resistance, water impermeability, perfect adhesion to different substrates, and providing at the same time excellent mechanical properties is highly desired.
As a matter of fact, fluoropolymer films known in the art have the drawback that mechanical stress on the film surface generates a haze effect that leads to a loss of the film transparency so that the operation of the solar cell is compromised. Since a solar cell module is normally exposed to a natural environment for a long time, it is mandatory that the material forming the protective film thereof has excellent hardness, abrasion resistance and impact strength so that it can protect the solar cell from any possible event and particularly from the elements (rain, hail, wind, sand, etc).
Moreover, some of the fluoropolymer protective films known in the art require either the use of adhesives or particular treatments in order to ensure their bonding to the solar cell.